1. Field of the Invention
The present invention relates to a cross coil used in a cross coil-type indicator, in particular, relates to an improved cross coil used in a cross coil-type indicator in which a magnetic rotor, to which a shaft of an indicating pointer is secured, is rotated by a magnetic field component of the cross coil.
2. Description of the Prior Art
Conventionally, an indicator shown in FIG. 1 is commonly known as this type of cross coil type indicator. In this indicator, a disk-shaped magnetic rotor 2 to which a shaft of an indicating pointer 1 is secured, is positioned in a rotatable manner in a coil bobbin 3 which can be separated into an upper and a lower section. On the outer periphery of the coil bobbin 3 there is wound a first coil windings 4 into which is inserted a bearing 3a of the coil bobbin 3 through which the shaft of the indicating pointer is passed. Next a second coil windings 5, into which is also inserted the bearing 3a of the coil bobbin 3, is wound on the outer periphery of the first coil 4 windings at about 90.degree. to the first coil windings 4. In addition, a scale 6 is installed on a bracket 3b of the coil bobbin 3 and an indicating pointer 7 is mounted to the shaft 1 on the top surface of the scale 6.
Specifically, as diagrammatically illustrated in FIG. 2, in the cross coil as described above the rotatable magnetic rotor 2 which comprises a permanent magnet is disposed in a magnetic field generated by the first and second coil windings 4, 5.
In this configuration, the magnetic field .phi..sub.1, .phi..sub.2 produced by the first and second coil windings 4, 5 are proportional to the product of the current flowing in the first and second coil windings 4, 5 and the number of turns in the windings. Therefore, if the number of turns for both the first and second coil windings 4, 5 is N and the currents flowing in the coil windings are I.sub.o sin .theta. and I.sup.o cos .theta. respectively, then the magnetic field component .phi. is in the direction of the vector component of the magnetic fields .phi..sub.1, .phi..sub.2 produced by the first and second coil windings 4, 5, and the magnetic field component is to describe a circle corresponding to a change of the angle .theta.. Accordingly, by setting the angle .theta. to the specified measured amount each of the magnetic fields .phi..sub.1, .phi..sub.2 becomes EQU .phi..sub.1 =I.sub.0 sin .theta..N (1) EQU .phi..sub.2 =I.sub.o cos .theta..N (2),
respectively.
The magnetic field component .phi. acts in the direction of the vector component of the magnetic fields .phi..sub.1 and .phi..sub.2, and its magnitude becomes ##EQU1## and by varying the angle .theta. to correspond to the specified measured amount as in FIG. 3, the direction of the magnetic field component .phi. corresponds to the specified measured amount. Accordingly the magnetic rotor 2 rotates in the direction of the component of the magnetic field, and as a result, by rotating in cooperation with an indicating pointer 7, the angle of rotation of the pointer 7 indicates the specified measured amount, which can be realized by a driver.
However, actually, in this type of conventional cross coil, as indicated in FIG. 4, first, the specified number of turns of the first coil 4 are wound, then, the specified number of turns of the second coil 5 are wound on the outside of the first coil 4 crossing those windings at 90.degree..
Therefore, in this type of conventional cross coil, as in FIG. 4, because the outer diameter of the first coil 4 which is wound on the inside differs from that of the second coil 5 which is wound on the outside with the same number of turns, the total length of the second coil 5 is the longer of the two. Accordingly, when the resistance per unit length of each coil is the same, the resistance of the second coil 5 is large in comparison with that of the first coil 4. Also, if the winding is performed so that the resistances of both coils are the same, the number of turns in the outside second coil 5 would be less in comparison with the number of turns in the first coil 4.
On the other hand, the magnitude of the magnetic fields .phi..sub.1 and .phi..sub.2 of the coils are proportional to the product of the current flowing in the coil and the number of turns of the winding, as described in equations (1) and (2). Accordingly, it is difficult for the conventional cross coil to equalize the maximum magnetic fields I.sub.o.N of the coils 4 and 5 which are produced to correspond to the specified measured amount (the angle .theta.). Specifically, when identical voltages are applied to the coil 4, 5, the currents flowing in each of the coils 4, 5 are different from each other so that the magnetic fields .phi..sub.1 and .phi..sub.2 are not identical.
For example, when the number of winding turns N is the same for each coil, the resistance of the outside second coil 5 is large, as outlined above, so that the currents I.sub.o in equations (1) and (2) are not the same, and the current in the first coil 4 becomes large in comparison with the second coil 5. If the currents in each coil are I.sub.1 and I.sub.2 respectively, the magnetic fields .phi..sub.1 and .phi..sub.2 become: EQU .phi..sub.1 =I.sub.1 sin .theta..N (4) EQU .phi..sub.2 =I.sub.2 cos .theta..N (5)
and I.sub.1 &gt;I.sub.2. Therefore, the magnitude of the magnetic field component .phi. becomes ##EQU2## and has only elliptic characteristics corresponding to the angle .theta. as shown in FIG. 5.
In this type of cross coil, for example, in the case of the current flowing for the angle .theta. of 45.degree. corresponding to the specified measured value, the magnetic fields .phi..sub.1 and .phi..sub.2 for the coils 4, 5, become, from equations (4) and (5): ##EQU3## The angle for this magnetic field component becomes ##EQU4## and does not agree with the angle (45.degree.) which corresponds to the specified measured value which should be indicated. Accordingly, in a conventional cross coil, because the direction of the magnetic field component of the cross oil does not agree with the direction of the angle .theta. which corresponds to the specified measured value, the problem arises that the specified measured value cannot be accurately displayed.